1. Field of the Invention
This invention relates to an image forming apparatus such as, e.g., a printer, a facsimile machine, an electrophotographic apparatus, a photocopier, or the like.
2. Description of Related Art
As shown in FIG. 18, an image forming apparatus is generally composed of a photosensitive drum 1 formed with an electrostatic latent image on a surface thereof, a developing roller 2 for shifting a toner 9 serving as a developer onto the photosensitive drum 1, a toner supplying roller 3 for shifting the toner 9 supplied arbitrarily from a toner cartridge 12 to the developing roller 2, a toner blade 10 for regulating a thickness of a toner layer shifted to the photosensitive drum 1, an LED (Light Emitting Diode) head 26 for forming the electrostatic latent image on the photosensitive drum 1, a charging roller 4 for rendering the surface of the photosensitive drum 1 have a prescribed electric potential, a transfer roller 5 for transferring the toner attached on the electrostatic latent image formed on the photosensitive drum 1 to a printing medium 11, and a cleaning blade 7 for removing the toner remaining after transferred on the surface of the photosensitive drum 1.
In consideration of maintainability, furthermore, it is general that the developing roller 2, the toner supplying roller 3, the toner blade 10, and the toner cartridge 12 are structured as an EP (Environmental Photocell) cartridge 13 to be replaceable as a united body.
As shown in FIG. 18, negative bias voltages, i.e., Vg, Vs, and Ve, are respectively applied to the developing roller 2, the toner supplying roller 3, and the charging roller 4 while a positive bias voltage is applied to the transfer roller 5. It is to be noted that since the subsequent description mostly explains the negative bias voltages applied to the developing roller 2, the toner supplying roller 3, and the charging roller 4, these bias voltages are to be explained as absolute value voltages with respect to the zero potential as a reference for the sake of convenience. For example, in a case of “the high potential,” the potential is more negative.
With the above described conventional image forming apparatus, the total potential amount of the toner per unit area, hereinafter referred to as a toner potential, may exceed an appropriate range with respect to a surface potential of the photosensitive drum 1, where the toner, the toner supplying roller 3, the developing roller 2, or the like changes charging property thereof according to an environmental condition such as, e.g., temperature and humidity of the toner cartridge 12, thereby greatly changing the toner amount per unit area attached near to the developing roller 2 even under application of the same bias voltage.
For example, where becoming superior in the charging property upon the change in the environmental condition, the toner is attached more to the developing roller 2, thereby heightening comparatively the potential thereof near the developing roller 2, and raising a problem that irregular printing may occur in causation of attachment of the toner to the photosensitive drum 1 at an unexposed area at which a printing is not implemented due to the high toner potential. Conversely, where becoming inferior in the charging property for the change in the environmental condition, the toner is attached less to the developing roller 2, thereby lowering comparatively the potential thereof near the developing roller 2, and raising a problem that toner concentration lowers, thereby causing the printing to become blurred.
With the conventional image forming apparatus, to cope with the change in the toner potential due to the change in the environmental condition as described above, the appropriate bias voltage of the toner supplying roller 2 corresponding to each environmental condition is previously determined according to, e.g., an experiment or the like, and the bias voltage is set at a time of the printing upon retrieving the bias voltage corresponding to the environmental condition detected with a temperature and humidity sensor or the like (see, e.g., Japanese Patent Application Publication No. 7-134477).
However, with the above described conventional image forming apparatus, there may raise a problem that where the temperature and humidity sensor is not placed at the appropriate position within the EP cartridge 13, differenced may occur as detection errors of the environmental condition, so that the appropriate bias voltage cannot be set, while where the charging property changes according to change over time or to each replacement of the toner cartridge 12, the bias voltage is set with the same setting table, so that the appropriate bias voltage cannot be set as well.